1. Field of the Invention
This invention relates generally to a semiconductor technique and more particularly to a silicon carbide film containing oxygen used as a barrier film for forming an interconnect structure or as an anti-refractive film for photolithography, which is formed by plasma polymerization.
2. Description of the Related Art
Under the plasma chemical vapor deposition method (plasma CVD method), a thin film such as an interlayer insulation film, passivation film or anti-refractive film is deposited on a semiconductor substrate in an atmosphere of 1 Torr to 10 Torr by placing the semiconductor substrate, being the processing target, on a resistance-heating heater or other heater that has been heated to a temperature of 50° C. to 550° C. The heater is placed on the opposite side of a shower plate that releases a reaction gas, and high-frequency power is applied to the shower plate to cause high-frequency discharge between the heater and shower plate and thereby generate plasma. Deposition of a thin silicon carbide film on a semiconductor substrate is implemented by applying a high-frequency power of 13.56 MHz to 60 MHz or other frequencies at 300 W to 1,000 W, as well as a low-frequency power of 400 kHz at 50 to 200 W, to the shower plate to cause plasma discharge between the heater and shower plate. Here, 4MS, 3MS, 1MS or other material is used for forming a silicon insulation film, while CO2, O2, NH3, N2 or alcohol is used as an additive gas to break down the process gas introduced as a result of discharge.
Semiconductor devices use Cu wires offering higher thermal durability and lower resistance than conventional Al wires, in order to solve the problem of greater RC delays caused by increasingly finer patterns. As the design dimensions of devices decrease, the dielectric constants of interlayer insulation films are becoming lower, and 130-nm devices are using SiOF films with a dielectric constant of approx. 3.4 to 3.7. The importance of resistance is also increasing with wiring materials, and manufacturers have shifted from Al (aluminum) wires to Cu (copper) wires offering lower resistance. However, Cu is subject to significant diffusion under heat, and thus requires a barrier film to block diffusion. Since conventional low-dielectric-constant films (low-k films) offer poor capability when it comes to blocking Cu diffusion, they do not provide effective barrier films. For this reason, silicon carbide films are used as barrier films.
Barrier films must satisfy numerous requirements, and a barrier film not satisfying all these requirements cannot be applied successfully. An appropriate thickness of silicon carbide film is determined after conducting a Cu diffusion test to confirm the region over which Cu is diffused. Since the dielectric constant of the entire wiring layer must be lowered, it is desirable to keep the silicon carbide film as thin as possible while suppressing Cu diffusion into the silicon carbide film. From the viewpoint of device performance, the required duration of thermal durability test is 14 hours in a 400° C. atmosphere. Since Cu also diffuses under application of electricity, the BTS test is conducted where the two causal factors—namely, heat and electricity—are applied. Passing the BTS test demonstrates the satisfaction by the tested film of the functional requirements for Cu-diffusion blocking film. As a simplified test, a sample comprising a silicon carbide film deposited on Cu is exposed to a 400° C. atmosphere for 4 hours. If the level of Cu diffusion into the silicon carbide film does not exceed 20 nm, the sample is considered acceptable.
The need for insulation films with a lower dielectric constant has made it a requirement for barrier films to also have a low dielectric constant, and silicon carbide films have been adopted as barrier films. With 100-nm devices, low-k films with a dielectric constant of approx. 2.9 to 3.2 are used as insulation films, while films made of SiCN, SiCO, etc., with a dielectric constant of approx. 4.5 to 5.0 are used as barrier films.
In particular, it is important that barrier films not only block Cu diffusion, but also block moisture penetration to prevent moisture absorption and consequent oxidization of the Cu wire in the bottom layer. In view of these requirements, N, O and other impurities are added to a pure silicon carbide film to improve moisture penetration resistance. When impurities are added, however, lowering the dielectric constant to 4.0 or below becomes difficult. With 65-nm devices where the dielectric constant required of the barrier film is 4.0 or below, silicon carbide films not containing impurities are being evaluated for many applications. When its dielectric constant decreases, however, a pure silicon carbide film becomes notably less stable over time compared to conventional SiCN and SiCO films. The most troublesome factor is that oxidization of the Cu surface accelerates due to moisture penetration, which is fatal in device applications. One plausible explanation is a lower film density of 1.1 to 1.3 kg/cm3, compared to 1.8 to 2.0 g/cm3 with SiCN and SiCO films. There is another area of concern that, because of this lower density, the effect of NH3 (ammonia) treatment used for Cu surface reduction might cause a problem in resist pattern forming. To improve these drawbacks, sometimes a film resistant to moisture penetration and also having a relatively high dielectric constant is used as a cover film both above and below of a bulk barrier film. Since barrier films used in devices must have a small thickness of 10 to 50 nm, however, existence of cover films above and below the barrier film will make the etching processing complex due to the control conditions needed to keep the cover films thin, and it will also increase the dielectric constant due to the multi-layer structure. In the meantime, one key factor in ensuring stability is to eliminate free bonds from the film. Therefore, when impurities are added to SiCN and SiCO films, it is necessary to add impurities to achieve stability. Traditional silicon carbide films made of 4MS and 3MS also present problems, such as achieving a low dielectric constant is difficult with SiCO and SiCN and process margins are narrow.
On the other hand, SiON-based films have traditionally been used as anti-refractive films for lithography. Since the mainstream lithography wavelength was 248 nm, the required film quality was determined by the specifications of lithography and the film used in the bottom layer. In other words, n (reflection factor) and k (extinction coefficient) determined from these factors were important. When the wavelength was 248 nm, there were less limitations on the resist and consequently less limitations on the anti-refractive film. However, times have changed and the wavelength used for lithography has also changed from 248 nm to 193 nm, accompanied by notable changes in processing structures. It is expected that this shorter lithography wavelength will change the resist material and ultimately present a new set of problems such as poisoning due to resist degradation.